Setting up of quadrature signals

ABSTRACT

A device to assist in setting up the pitch, roll, yaw and standoff of a read head relative to a scale, so as to improve the quadrature relationship of its outputs. Two superimposed Lissajous figures are produced on an oscilloscope screen. One of these figures is a rotation or reflection of the other. The read head is adjusted until the Lissajous figures coincide. Simpler, less accurate arrangements are also described, in which a variable DC output signal is produced which represents the radius of a hypothetical Lissajous derivable from the signals. The read head is then adjusted to give a constant DC output, representing a constant Lissajous radius.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatus to assist in the settingup of devices which produce output signals in quadrature, for example soas to ensure that the output signals are accurately in a 90° phaserelationship, or accurately of the same amplitude, or accurately withzero DC offsets.

Such a method and apparatus are useful in setting up a scale andreadhead, such as used on co-ordinate measuring machines or machinetools, to determine the position of a movable part of such a machinerelative to a fixed part. It is well known to provide such scales andreadheads wherein the readhead has two outputs in quadrature. Theseoutputs can be fed to further circuitry for counting the cycles of theincoming signals so as to determine position, for determining thedirection of movement by determining which of the quadrature signalsleads the other, and/or for providing position interpolation within onecycle of the output. Of course, it is desirable that the readhead shouldbe set up correctly aligned relative to the scale such that the outputsare accurately in quadrature, with a 90° phase shift, the sameamplitudes and with zero DC offsets. Particularly in the case where thesignals are to be fed to an interpolator, this can affect the accuracyof the resulting determination of the relative position.

An example of a scale and readhead providing quadrature outputs to aninterpolator is shown in European Patent Application No. EP-0274841-A.

One way to determine such factors would be to feed the two quadraturesignals to the X and Y inputs of an oscilloscope. This produces aLissajous figure, which should be a perfect circle centred on zero ifall of the above factors are correctly set up. The mechanical positionof the readhead relative to the scale can be adjusted to achieve this.However, it is very difficult to judge whether the Lissajous figureobserved is perfect. The results also depend upon the accuracy ofcalibration of the oscilloscope, for example ensuring that the gains ofits X and Y channels are exactly matched and that neither introduces aDC offset.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a method of setting up adevice which provides two signals in quadrature, comprising:

processing said quadrature signals in a processing circuit which has atleast one output line, said processing circuit including means forcombining the quadrature signals, to give at least one combined outputsignal on said at least one output line, representative of the alignmentof the quadrature signals;

passing said at least one combined output signal to a display device anddisplaying an indication of said alignment of the quadrature signals;and

adjusting said device which provides the quadrature signals to improvethe indicated alignment.

A second aspect of the present invention provides a method of setting upa device which provides two signals in quadrature, comprising producingon an oscilloscope screen two superimposed Lissajous figures derivedfrom the quadrature signals, one of the Lissajous figures being arotation or reflection of the other, and adjusting the device so as toimprove the co-incidence of the Lissajous figures.

Further aspects of the invention provide apparatus for use in performingthe above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block schematic diagram of a first embodiment, connected toa scale and readhead,

FIGS. 2 and 3 are circuit diagrams of two further embodiments, and

FIG. 4 is a waveform diagram for the circuit of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is a scale 10 along which a readhead 12 canbe traversed, producing SIN and COS outputs 14, 16 which are (at leastnominally) in quadrature. When installing the scale 10 and readhead 12on a machine such as a co-ordinate measuring machine or machine tool, itis required to adjust their relative alignments so as to give outputs14, 16 which are accurately at 90° to each other, with the sameamplitudes and each with zero DC offsets. This is done by adjusting theroll, pitch, yaw and standoff between the readhead and the scale.

To assist such alignment, the outputs 14, 16 of the readhead 12 aretaken via circuitry 18 to the X and Y inputs of an oscilloscope 20.

The circuity 18 includes an inverter 22 which produces a signal -COS ona line 24 which is an inverted version of the COS signal 16.

These signals are fed to the oscilloscope 20, as described below, via amultiplexer 26. The multiplexer 26 is driven by a continuously cyclingcounter/oscillator 28, and has two channels, one feeding the X input ofthe oscilloscope and one feeding the Y input. Each channel has fourinputs which are connected in turn tc the respective oscilloscope input.The rate at which this multiplexing takes place is controlled by twobinary output lines 38 of the counter/oscillator 28, giving thenecessary four time slices.

In a first time slice, the COS signal 16 is fed to the X input of theoscilloscope and the SIN signal 14 is fed to the Y input. This producesa first Lissajous FIG. 30 on the screen of the oscilloscope (assumingthe readhead to be moving relative to the scale; if it is stationary asingle dot is produced). During a second time slice, the multiplexer 26connects the SIN signal 14 to the X input of the oscilloscope, and the-COS signal 24 to the Y input. This produces a second Lissajous FIG. 32on the oscilloscope screen. With the inputs as just described, it can beshown that the second Lissajous FIG. 32 is the same as the firstLissajous FIG. 30, but rotated through 90°. Alternatively, the secondLissajous FIG. 32 can be considered as being derived from the first byreflecting the first Lissajous FIG. 30 twice, firstly about a 45° axisand secondly about a vertical axis. These two alternativetransformations from the first Lissajous FIG. 30 to the second LissajousFIG. 32 can be shown to be theoretically equivalent.

The frequency of the counter/oscillator 28 is sufficiently high that thetime slices are indistinguishable to the naked eye on the oscilloscopescreen, so that the two Lissajous figures appear superimposed on top ofeach other as shown.

If the readhead is perfectly aligned relative to the scale 10, then boththe two Lissajous figures will be perfect circles, centered on zero, andco-incident with each other. If the alignment is not correct, then thisperfect situation will not be the case. It is an easy matter for theuser to adjust the alignment (roll, pitch, yaw and standoff) until thetwo Lissajous figures are co-incident. This is done by moving thereadhead along the scale, while observing the oscilloscope, adusting thealignment, then repeating the process as necessary.

An advantage of this technique is that no reliance is placed upon theaccuracy of calibration of the oscilloscope 20. If the oscilloscope 20itself introduces mis-matches of gain between the two channels, or DCoffsets, then the resulting Lissajous figures will no longer be perfectcircles centred on zero. Nevertheless, all the user has to do is alignthe readhead relative to the scales such that the (imperfect) Lissajousfigures are co-incident with each other, and he can then be confidentthat the alignment is correct, and that the imperfections are introducedby the oscilloscope and not by misalignment of the readhead.

The discussion above has assumed that the output signals 14,16 from theread head 12 are sinusoidal. However, it is a further advantage of thepresent technique that it also works with symmetrical non-sinusoidalsignals, e.g. triangular or square signals in quadrature. The Lissajousfigures will no longer be circles or ellipses, but it is still easy toalign the read head so that they coincide.

The block schematic diagram also shows certain features which, while notessential, are of assistance during the setting up procedure.

For example, the multiplexer has a third time slice during which itconnects both X and Y inputs of the oscilloscope to zero volts (line34). This produces a dot 36 at the centre of the oscilloscope screen,superimposed on the Lissajous figures. If the oscilloscope isintroducing gross DC offsets in one or both channels, the user caneasily adjust it so as to centre the dot 36. However, it is notessential to do this accurately.

Secondly, in addition to the binary outputs 38 which control themultiplexer 26 and cause it to cycle through its four time slices, thecounter/oscillator 28 may have two further binary outputs 40. These aretaken from less significant bits of the binary outputs of the counter,and thus are at a higher frequency than the outputs 38. Thus, the twobinary outputs 40 can cycle through all four possible binary valuesduring the fourth time slice of the multiplexer 26 (and indeed may do somore than once). During the fourth time slice, the two outputs 40 arerespectively connected to the X and Y inputs of the oscilloscope, viarespective voltage dividers R4, R7 and R5,R6. These voltage dividersprovide positive and negative reference voltages (as the outputs 40 ofthe counter switch between positive and negative values). The X and Yinputs of the oscilloscope oscillate between these positive and negativevalues (one at twice the rate of the other) during the fourth timeslice. The result is to produce four dots 42, which if the oscilloscopegain is correctly set will be at the four corners of a square fillingthe majority of the oscilloscope screen. This assists the user to set upthe gains of the two oscilloscope channels to appropriate values. Again,it is not essential that they should be absolutely correct or absolutelymatched. The values of the resistors R4,R7,R5, R6 are chosen such thatthe dots 42 represent the maximum amplitudes delivered on the lines 14and 16 when the readheads are saturated. This makes it easy for the userto see that he is getting a good signal level from each of the outputsof the readhead, without going over the saturation limits (which wouldresult in distortion of the resorting COS and SIN waveforms).

An optional switch 44 may be provided to inhibit the counter/oscillator28 and force it to produce a zero output. The multiplexer 26 then isconstrained to feed the SIN and COS outputs 14, 16 directly through tothe X and Y inputs of the oscilloscope 20, thereby producing aconventional Lissajous figure for monitoring purposes.

In the example described, the Lissajous FIG. 32 is rotated 90°, comparedwith the Lissajous FIG. 30. However, although this is the preferredembodiment, it is not the only way of producing useful results. TheLissajous FIG. 30 may be rotated through other angles, or may simply bereflected about an axis. For example, in another embodiment (not shown),during the first time slice of the multiplexer 26 the COS signal 16 isfed to the X input of the oscilloscope and the SIN signal 14 is fed tothe Y input, as in the embodiment described above. In the second timeslice, however, these signals are simply reversed, with the SIN signal14 being fed to the X input and the COS signal 16 to the Y input. Withsuch an arrangement, the second Lissajous FIG. 32 is simply a reflectionabout a 45° axis of the first Lissajous FIG. 30. In a furthermodification of this arrangement, the SIN and COS signals fed to themultiplexer for the first time slice are DC coupled to the multiplexer,so that the Lissajous figure is subject to DC offsets, while the SIN andCOS signals fed for the second time slice are AC coupled so as to removesuch DC offsets. This has the advantage that once the readhead 12 hasbeen aligned such that the Lissajous figures are co-incident, it followsthat any DC offsets must have been removed. Of course, such AC couplingis not necessary in the preferred embodiment as described above.

FIG. 2 shows a device which is simpler to use than that of FIG. 1, sinceit provides a quick indication of whether the sine and cosine signalsSIN and COS from the readhead are correctly set up. However, theindication given is not as accurate as the embodiment of FIG. 1, so thisembodiment primarily finds use in lower accuracy applications where easeof set up is of prime importance.

If one considers the hypothetical Lissajous figure which would beproduced by feeding the SIN and COS signals to the X and Y inputs of anoscilloscope, for example the Lissajous FIG. 30 shown in FIG. 1, it canbe seen that the radius of the Lissajous figure from the origin 36 willbe constant throughout the cycle of the incoming waveforms, when theinputs are correctly adjusted. The circuit of FIG. 2 is designed to givea simple indication corresponding to the radius of the Lissajous.

For this purpose, the inputs SIN and COS are taken to respectiveanalogue squaring circuits 46,48 which produce signals SIN² and COS².These are added together in an adder 50, and the square root of theresult is then taken by a circuit 52. The resulting signal on a line 54is proportional to the magnitude of the radius of the hypotheticalLissajous figure discussed above. This is taken to a standard integratedcircuit bar graph driver 56, which, at any instant of time, lights oneof a series of light emitting diodes (LEDs) 58 correponding to themagnitude of the signal 54. Thus, at any instant, one of the LEDs 58will be lit, corresponding to the instantaneous magnitude of the radius.

In use, the readhead 12 is driven along the scale 10 so as to producethe sinusoidal inputs SIN and COS. If these inputs are accurately inquadrature, accurately at 90° phase separation, and with no DC offsets,then the radius signal 54 will be a constant DC level, and only one ofthe LEDs 58 will light up during the movement of the readhead 12.However, where the alignment is not accurate, the signal 54 will not beconstant, but will have superimposed upon it an oscillation at thefrequency of the input signals. Over a cycle of the input signals,therefore, several of the LEDs 58 will light up, and at normal speeds ofmovement of the readhead 12 they will appear to be lit simultaneously.

Adjustment of the scale/readhead alignment, therefore, is as follows.Firstly, the standoff of the readhead from the scale is adjusted so asto get a strong DC signal 54 (indicated by the lighting of a LED 58which is relatively high in the series of LEDs). This is done with thereadhead stationary. The lower value LEDs in the series 58 may becoloured red, and the rest green, so as to indicate when the DC level ofthe signle 54 is adequate. Then, the readhead is moved, and (assumingmisalignment) several of the LEDs 58 will light simultaneously asdescribed above. The roll, pitch and yaw of the readhead are adjusted toreduce this, and the process is repeated until satisfactory results areobtained.

Although the embodiment of FIG. 2 is simpler to use than that of FIG. 1,its disadvantage is that the circuits 46,48 and 52 are quite complex,and therefore expensive to implement. FIG. 3 therefore shows a simplercircuit which performs the same job.

FIG. 3 has a bar graph driver 56 and a series of LEDs 58, as in FIG. 2.The difference lies in the circuits used to produce the signal 54 whichrepresents the radius of the hypothetical Lissajous figure.

In FIG. 3, the inputs SIN and COS are taken to a number of inverters,adders and subtractors, 60 to 70, which perform various inversions andcombinations of the SIN and COS signals. The resulting signals aretaken, with the original SIN and COS signals, to an analogue gate 72,which serves to select whichever of its inputs is at the most positivevalue at any given instant, and to pass that signal through to theoutput line 54 and on to the bar graph driver 56.

Specifically, a -SIN signal is produced by inverting the SIN signal inan inverter 60. A signal SIN+COS is produced by adding the SIN and COSsignals in an adder 62. The adder 62 scales the signal by a factor 0.7,so that it has the same nominal amplitude as the inputs SIN and COS. Aninverted version -(SIN+COS) is produced by an inverter 64. A signal -COSis produced from the COS signal by an inverter 66. A signal SIN-COS isproduced by subtraction in a substractor 68, which again scales thesignal by 0.7. A signal -(SIN-COS) is produced from this by inversion inan inverter 70.

The various signals which are input to the gate 72 are shown in FIG. 4,and it will be seen that each is phase shifted relative to the originalSIN and COS signals. If the original SIN and COS signals resulted from aperfectly aligned readhead 12, then each of these signals would have thesame amplitude, but any inaccuracies in the phase separation, amplitudesor DC levels of the incoming SIN and COS signals will result in thevarious inputs to the gate 72 having different amplitudes.

The effect of the gate 72, which always selects the most positive of theincoming signals, is shown by the signal 54 in FIG. 4. If all the inputsignals have the same amplitude, then the signal 54 is subject only tominor fluctuations between the peak of one input signal and the next.However, if the incoming signals to the gate 72 have differentamplitudes, a larger variation is superimposed on top of these minorfluctuations, as shown by broken lines in FIG. 4. This larger variationcorresponds to the variation in the radius of the hypotheticalLissajous, as with the FIG. 2 embodiment, and is caused by the originalSIN and COS signals having differing amplitudes, or DC offsets, ornon-90° separation.

The result, when the signal 54 is fed to the bar graph driver 56, isthat if the input signals to the gate 72 all have the same amplitude,then only one of the LEDs 58 (or at most two) will be lit, because theminor fluctuations in the signal 54 are less than the difference betweentwo adjacent thresholds within the bar graph driver. However, if theinput signals to the gate have different amplitudes, then severaldifferent LEDs in the series 58 will light up, at different parts of thecycle. As the readhead is driven along the scale 10, these differentLEDs appear to light simultaneously.

The method of use of the circuit of FIG. 3 is therefore the same as thatof FIG. 2. After adjusting the standoff to give a good DC level for thesignal 54, the readhead 12 is moved over the scale. Normally severalLEDs 58 will light. The readhead is now adjusted, and the procedurerepeated until only one or two LEDs remain alight as the readhead moves.

I claim:
 1. A method of setting up a device which provides two signalsin quadrature, comprising producing on an oscilloscope screen twosuperimposed Lissajous figures derived from the quadrature signals, oneof the Lissajous figures being a rotation or reflection of the other,and adjusting the device so as to improve the co-incidence of theLissajous figures.
 2. Apparatus for setting up a device which providestwo signals in quadrature, comprising a processing circuit having twoinput lines for receiving the quadrature signals and two output linesfor connection to X and Y inputs respectively of an oscilloscope, theprocessing circuit including means for combining the quadrature signalsto produce two combined output signals, one on each of the output lines,the combination being such as to produce two superimposed Lissajousfigures derived from the quadrature signals on a screen of anoscilloscope when connected to the output lines, one of the Lissajousfigures being a rotation or reflection of the other.
 3. Apparatusaccording to claim 2, wherein the combining means has means for feedingthe quadrature signals to the output lines in at least two separate timeslices, thereby producing said two superimposed Lissajous figures. 4.Apparatus according to claim 3, including means for inverting one of thequadrature signals in one of the time slices.
 5. Apparatus according toclaim 3, including means for feeding DC levels to the output lines in atleast one further time slice, thereby producing at least one referencedot on the oscilloscope screen.
 6. Apparatus according to claim 5,wherein said DC levels are zero.
 7. Apparatus according to claim 5,wherein said DC levels represent saturation levels of the quadraturesignals.
 8. Apparatus according to claim 4, including means for feedingDC levels to the output lines in at least one further time slice,thereby producing at least one reference dot on the oscilloscope screen.9. A method of setting up a device which provides two signals inquadrature, comprising:processing said quadrature signals in aprocessing circuit which has two output lines, said processing circuitincluding means for combining said quadrature signals to produce twocombined output signals on said at least two output lines, said outputsignals being representative of the alignment of the quadrature signals;passing said two combined output signals to X and Y inputs of anoscilloscope having a screen, two superimposed Lissajous figures derivedfrom said quadrature signals and representative of the alignment thereofbeing displayed on said screen, one of said Lissajous figures being arotation or reflection of the other; and adjusting said device whichprovides the quadrature signals to improve the coincidence of saidLissajous figures.
 10. Apparatus for setting up a device which providestwo signals in quadrature, comprising:a processing circuit, having twoinput lines for receiving the quadrature signals and an output line, theprocessing circuit including means for combining the quadrature signalsto produce an output signal representing the radius of a hypotheticalLissajous figure derivable from the quadrature signals, said combiningmeans including a plurality of circuit means for producing respectivedifferent phase-shifted inversions and/or combinations of the quadraturesignals; and an analogue gate means for selecting whichever of theoutputs of said circuit means is the highest; and a display deviceconnected to said output line for receiving said output signal andproducing therefrom an indication of the alignment of the quadraturesignals, said selected output being passed to said display device. 11.Apparatus according to claim 10, wherein the display device comprises abar graph driver.